Hypothetically speaking telling the difference between mental health problems and demonic possession can be difficult unless there is some sort of physical indicator, change of eye colour, jumpy awkward and uncordinated movements for example can be a sign that someone is being possessed and also controlled and that the entity possessing them may be having some difficult adjusting to its "fleshy" host, rapid change in personality speech patterns and behaviour can also be an indicator of possession but it could easily be some other entirely normal mental health problem. Those demonically posessed can show an aversion to holy things, crosses chruches etc... wont enter hallowed ground and can often be extremely hostile to those of a religous nature.